The present invention generally relates to optical semiconductor devices and more particularly to an optical semiconductor device including a laser diode operable in a 0.6 xcexcm wavelength band.
The optical wavelength band of 0.6 xcexcm is used extensively in storage devices such as an optical disk drive or a magneto-optical disk drive for optical writing or reading of information. Further, the optical wavelength band of 0.6 xcexcm is important in optical telecommunication that is conducted by using plastic optical fibers.
Thus, intensive investigations are being made in relation to a laser diode of an AlGaInP system that produces an output optical beam with the optical wavelength band of 0.6 xcexcm. The laser diode using the AlGaInP system is also important in color display devices as an optical source of red to green colors. It should be noted that the AlGaInP system is a III-V material providing the largest bandgap (2.3 eV or 540 nm wavelength) while simultaneously maintaining a lattice matching with a GaAs substrate.
On the other hand, such a laser diode using the AlGaInP system for the active layer thereof suffers from the problem of poor confinement of carriers, particularly electrons, in the active layer. More specifically, carriers escape easily from the active layer to adjacent upper and/or lower cladding layers due to the small band discontinuity formed at the heterojunction interface between the AlGaInP active layer and the adjacent cladding layers. Associated with such a small band discontinuity and resultant weak carrier confinement, the conventional AlGaInP laser diodes have suffered from the problem of large temperature dependence for the threshold characteristic of the laser oscillation. This problem of poor temperature characteristic of the laser diode is pronounced further when the bandgap of the active layer is increased for decreasing the laser oscillation wavelength by using a quantum well structure for the active layer.
In order to avoid the problem of overflowing of the carriers away from the active layer, the Japanese Laid-Open Patent Publication 4-114486 describes the use of a multiple quantum barrier (MQB) structure for the carrier blocking layer. Further, Hamada, H. et al., Electronics Letters, vol.28, no.19, Sep. 10th 1992, pp.1834-1836, describes the use of a strained MQW structure strained with a compressive stress. According to Hamada et al., op. cit., a continuous laser oscillation with a wavelength of as small as 615 nm is achieved by forming the strained MQW structure by using a quantum well layer having a composition of (Al0.08Ga0.92)0.45In0.55As in combination with a barrier layer and a GaAs substrate. However, the laser diode of thus produced has an unsatisfactory temperature characteristic, indicating that the desired, effective confinement of carriers is not realized.
Further, there is another proposal of a laser diode operable in the 600 nm wavelength band by using the material system of AlGaInP in combination with a substrate other than GaAs. For example, the Japanese Laid-Open Patent Publication 6-53602 proposes the use of an MQW structure including GaInP quantum well layers and GaInP barrier layers for the active layer in combination with a GaP substrate and AlGaP cladding layers. The foregoing reference further teaches the use of N as an impurity element forming an isoelectronic trap. This device, however, cannot provide the satisfactory confinement of carriers in the active layer. Thereby, the laser diode is characterized by a poor temperature characteristic.
Further, Japanese Laid-Open Patent Publication 7-7223 describes a laser diode operable in the wavelength band of 600 nm by using a III-V material containing N, such as InNSb or AlNSb in combination with a Si substrate or a GaP substrate. According to the reference, it becomes possible to form the laser diode on a Si substrate or a GaP substrate by incorporating N into such a III-V material. In the foregoing prior art, a composition of AlN0.4Sb0.6 is proposed as a lattice matching composition to the Si substrate, wherein it is described that a bandgap energy of about 4 eV corresponding to a ultraviolet wavelength band is obtained at such a lattice matching composition.
Unfortunately, such a III-V material system containing N generally shows a severe bowing in the bandgap due to the large electronegativity of N, and the desired increase of the bandgap is not achieved in the foregoing lattice matching composition, contrary to the prediction of the foregoing Japanese Laid-Open Patent Publication 7-7223. Further, in view of the existence of extensive immiscibility gap in the III-V material system containing N, formation of a III-V crystal containing such a large amount of N is not possible even when a non-equilibrium growth process such as MBE process or MOCVD process is used.
Thus, it has been difficult to achieve the laser oscillation at the 600 nm wavelength band even when other material systems are used. The use of the AlGaInP system, on the other hand, cannot provide the desired efficient confinement of carriers in the active layer due to the insufficient band discontinuity at the heterojunction interface between the active layer and the cladding layer.
Accordingly, it is a general object of the present invention to provide a novel and useful laser diode operable in the 600 nm wavelength band wherein the problems are eliminated.
Another and more specific object of the present invention to provide a laser diode operable in the 600 nm wavelength band with effective confinement of carriers in the active layer of the laser diode.
Another object of the present invention is to provide a laser diode, comprising:
a substrate of a first conductivity type;
a first cladding layer having said first conductivity type, said first cladding layer being formed on said substrate epitaxially;
an active layer of a group III-V compound semiconductor material formed epitaxially on said first cladding layer;
a second cladding layer having a second, opposite conductivity type, said second cladding layer being formed on said active layer epitaxially;
a first electrode injecting first type carriers having a first polarity into said active layer; and
a second electrode injecting second type carriers having a second, opposite polarity into said active layer,
said active layer having a composition of GaInNP containing therein N as a group V element.
According to the present invention, a large band discontinuity is guaranteed at the interface between the active layer and the first or second cladding layer as a result of the use of GaInNP for the active layer, and the efficiency of carrier confinement is improved substantially. By adjusting the amount of N in the GaInNP active layer, it becomes possible to set the band offset at the interface between the active layer and the first or second cladding layer as desired. Thereby, the laser diode shows an excellent temperature characteristic and operates stably at the room temperature environment. Further, as a result of the use of GaInNP for the active layer, the laser diode operates in the visible wavelength band including the 600 nm band. As the active layer of GaInNP is free from reactive Al, the growth of the active layer is conducted easily, without inducing island growth or associated problem of deterioration of crystal quality.
Another object of the present invention is to provide a vertical-cavity laser diode, comprising:
a substrate having a first conductivity type;
a first optical reflector provided on said substrate;
a first cladding layer having said first conductivity type on said first optical reflector in an epitaxial relationship with said substrate;
an active layer of a group III-V compound semiconductor material formed epitaxially on said first cladding layer;
a second cladding layer having a second, opposite conductivity type on said active layer in an epitaxial relationship with said active layer;
a second optical reflector provided on said second cladding layer;
a first ohmic electrode provided in ohmic contact with said substrate; and
a second ohmic electrode provided in ohmic contact with said second cladding layer;
said active layer having a composition of GaInNP containing therein N as a group V element.
According to the present invention, an efficient vertical cavity laser diode operable in the visible wavelength band is obtained. As a result of use of GaInNP for the active layer, a large band discontinuity is guaranteed at the interface between the active layer and the first or second cladding layer, and the efficiency of carrier confinement is improved substantially. By adjusting the amount of N in the GaInNP active layer, it becomes possible to set the band offset at the interface between the active layer and the first or second cladding layer as desired. Thereby, the laser diode shows an excellent temperature characteristic and operates stably at the room temperature environment. Further, as a result of the use of GaInNP for the active layer, the laser diode operates in the visible wavelength band including the 600 nm band. As the active layer of GaInNP is free from reactive Al, the growth of the active layer is conducted easily, without inducing island growth or associated problem of deterioration of crystal quality.
Another object of the present invention is to provide a method of fabricating a compound semiconductor device, comprising the step of:
(a) forming a first group III-V compound semiconductor layer epitaxially on a substrate;
(b) exposing a surface of said first group III-V compound semiconductor layer to an atmosphere containing N;
(c) forming, after said step (b), a second group III-V compound semiconductor layer on said first group III-V compound semiconductor layer epitaxially, said second group III-V compound semiconductor layer containing therein N as a group V element,
wherein said atmosphere is substantially free from a group III element.
According to the present invention, a part of the atoms constituting the group V element of the first group III-V compound semiconductor layer are replaced with N, and the epitaxial growth of the second group III-V compound semiconductor layer on the first group III-V compound semiconductor layer is facilitated substantially.
Other objects and further features of the present invention will become apparent from the following detailed description of the invention when read in conjunction with the attached drawings.